ErbB2 (ErbB2/HER2/Neu) is a member of the ErbB family of receptor tyrosine kinases, a family that also includes the Epidermal Growth Factor (EGF) Receptor (EGFR/ErbB1), ErbB3 (HERS), and ErbB4/HER4. Increased expression and signaling by ErbB2/HER2/Neu is observed in a significant fraction of mammary, ovarian, and prostate cancer patients, where they are correlated with tumor metastasis and poor prognosis. Consequently, our goal is to develop a positron emission tomography (PET) imaging agent that can be used to detect tumor cells that overexpress ErbB2. Such an agent could be used to noninvasively detect primary and metastatic tumor cells that overexpress ErbB2. Such an agent is not currently available. Our strategy is to express the extracellular domain of ErbB2 as a recombinant, soluble, epitope-tagged protein (s-ecErbB2). We will then couple s-ecErbB2 to a support matrix to create an affinity reagent that can be used to screen for molecules that bind s-ecErbB2. We will use this affinity reagent to screen a library of random, single-stranded oligodeoxynucleotides for those oligos (aptamers) that bind to s- ecErbB2. These aptamers will be radiolabeled and assayed for specific binding to ErbB2 overexpressed in a cultured cell line. Ultimately, these experiments should lead to the generation of 18F-labeled, nuclease-resistant aptamers that may be useful PET imaging agents. However, the generation and testing of 18F-labeled aptamers is beyond the scope of this proposal. Thus, the two specific aims of this proposal are to (1) express s- ecErbB2 and immobilize it on a chromatography support matrix; and (2) use this affinity reagent to screen a library of random DNA oligonucleotides for those aptamers that specifically bind s-ecErbB2 and assay the selected aptamers for binding to ErbB2 that is overexpressed on the surface of cultured cell lines.